You can do everything else right, but if you don’t invest in good soil, your garden won’t reach its full potential. The key to building good soil is using soil amendments that are readily available at most garden supply stores.
A soil amendment is anything you add to the soil to make it a better plant-growing medium. Soil amendments can add nutritive value, improve soil texture and aeration, or aid in water retention.
Compost, manure, worm castings, leaf mold, and biochar are a few examples of animal and plant-based soil amendments that enrich the soil with organic matter. Azomite, dolomitic limestone, greensand, and gypsum are several mineral-based soil amendments that add essential nutrients to the soil.
Keep reading for a detailed explanation of what soil amendments to use and why, as well as when to apply soil amendments to maximize your harvests.
Why soil amendments are essential
Talk to any grower who’s been at it for a few years, and they’ll tell you that building good soil is the surest ingredient to success. To get the healthiest plants and have the most abundant harvests, you need to grow in a nutrient-rich, well-draining growing medium. Plants that are starved of certain nutrients will struggle to flower and bear fruit, and plants without a healthy balance of moisture and air in their roots will succumb to detrimental diseases.
Most growers don’t naturally have ideal soil for growing vegetables, flowers, and herbs. The ideal soil type is fluffy, nutrient-rich loam, but most folks have either sandy or clay soil. This article explains the different types of soil and will guide you in finding out what kind of soil you have in your garden.
Soil amendments can be either organic or inorganic. Organic soil amendments come from a source that was at one time, or is still currently, living. Compost, manure, and biochar are considered organic soil amendments since they are biological matter. Inorganic amendments are made of minerals—lime, perlite, and azomite are considered inorganic amendments.
Organic soil amendments
Organic soil amendments are derived from once-living organisms and can include:
1. Compost
High-quality compost adds more than just essential nutrients to the garden. Organic matter (like compost, grass clippings, and manure) also improves soil texture. Organic matter tends to have larger soil particles that improve drainage and aeration, especially in clay soils.
You can buy compost in bags at your local nursery or garden supply store or make your own from kitchen scraps! But compost is only effective as a garden amendment when it’s composted. Don’t go throwing whole pieces of vegetables in your garden—that won’t accomplish anything and will probably just draw animals to your garden.
2. Manure
Manure is another organic amendment that can benefit your plants when used correctly and harm them if used incorrectly. Animal manure, whether from cows, chicks, or other livestock, must be composted before it can be applied to the garden. And unlike other soil amendments, manure has a very particular window when you can safely use it. Manure must be applied to the garden 90 days before harvest or about two weeks before planting.
3. Worm castings
Often called “black gold” for how rich it is, worm castings, or vermicompost, is essentially worm excrement. Gardeners treasure worm casting for its high concentration of nitrate, a biodegradable form of nitrogen.
4. Leaf mold
Leaf mold is compromised of chopped leaves that have been composted. It’s easy to make at home—just stay away from leaves from walnut trees, as these can leech a chemical that is a growth inhibitor for many plants.
5. Peat moss or coco coir
If you happen to have bags of unused peat moss or coco coir, you can add those to your native soil to improve its texture. Peat moss and coco coir can be expensive soil amendments when purchased in bulk, but even a small amount can be effective. Coco coir is a more sustainable alternative than Sphagnum peat moss, which is mined from Canadian bogs.
6. Biochar
Essentially charcoal, biochar is made by burning organic matter like animal bones and woody plant material. Biochar is an excellent source of carbon and phosphorus.
Inorganic mineral amendments
Organic soil amendments are just one piece of the puzzle of your garden’s health—plants also need minerals that aren’t as readily available through organic matter. This is where minerals come into play.
7. Azomite
The name is really an acronym for “A to Z Minerals with Trace Elements.” Azomite is a volcanic dust mined in Utah that contains about 60 trace elements and minerals.
8. Dolomitic limestone
An enriched type of limestone that raises the pH of alkaline soil and adds calcium and magnesium.
9. Greensand
Also called glauconite, greensand is an excellent soil amendment for clay soils. Greensand adds potassium, iron, and magnesium to the soil and improves texture.
10. Gypsum
Part calcium and part sulfur, gypsum is an excellent mineral for detoxifying the soil and removing excess sodium.
11. Perlite
Horticultural perlite begins as volcanic glass and is then heated until the particles expand in the styrofoam-like crumbles commonly found in potting soil. Perlite improves soil texture by adding drainage and aeration.
12. Vermiculite
Often paired with perlite, vermiculite is another mineral that looks and acts quite differently. Vermiculite has a flaky consistency, and rather than help aerate the soil, vermiculite actually retains moisture like a sponge.
There are many more mineral amendments, but some combination of the above will add plenty of nutritional value for most gardens.
A word on fertilizers
Fertilizer is technically a soil amendment since it is added to the soil. But fertilizers differ from soil amendments because fertilizers feed the current season’s plants rather than build good soil over time.
Synthetic fertilizers are often less expensive than organic fertilizers and tend to bring more immediate results. However, organic fertilizers are more likely to benefit the soil long-term, while synthetic fertilizers can potentially damage the soil due to high amounts of salt. You’re better off playing the long game and building good soil with amendments, but when you do reach for a fertilizer, opt for organic.
Nutrient needs for plants
Just like people, plants have nutritional needs.
- Nitrogen (N) is used by plants to grow stems and leaves. The amount of nitrogen in the soil directly affects plant growth—too little nitrogen results in stunted plants, but too much nitrogen will result in excess growth and leggy plants.
- Phosphorus (P) is an essential nutrient for plant photosynthesis and aids in developing flowers, fruit, and roots.
- A healthy balance of potassium (k) allows plants to regulate their water uptake and photosynthesis processes. Potassium also contributes to healthy root systems and flower and fruit production.
Magnesium (Mg), calcium (Ca), and sulfur (S) are considered secondary nutrients—still essential to plant growth and development but not used in as high amounts. The proper soil amendments usually add enough of these secondary nutrients.
Amending garden soil: the timeline
Building good soil isn’t as simple as throwing down bags of limestone and fertilizer and hoping for the best. Before you can do anything, you need to understand the chemical makeup of your soil so you can adjust accordingly.
First, take a soil test
Before you add anything to your soil, organic or otherwise, you should have your soil tested. This is easy to do—all you need to do is take a soil sample and mail it or bring it to your local university extension office. They’ll mail you back a detailed report explaining what nutrients your soil already has, what it needs, and your native soil’s pH levels.
Talk to your local extension agent for specific instructions, but taking a soil test is pretty straightforward. The key is to dig down at least six inches and only collect soil from the bottom four inches—try not to get any weeds or topsoil in the mix. Then, dig another hole in a different place in your garden. If your garden is big enough and you want to know the makeup of different plots, keep the samples separate. If you have a smaller garden, combine the samples into one bag and send the mix off to be tested.
It’s always a good idea to send a soil test off to a lab, but if you don’t have time to wait for your results or want to see the process for yourself, you can conduct your own soil test with our Soil Analyser. This combination pH and fertilizer analyzer reads soil pH and nutrient levels. To test your soil’s pH level, simply insert the clean prongs into the ground and wait for a reading.
Next, understand the results of the soil test
Your soil test results will likely contain several categories and corresponding numbers. You want soil pH to hover somewhere around 7 (neutral) or a little lower, like 6.5 (slightly acidic). Most plants prefer soil that is slightly acidic to neutral.
The University of California Agriculture and Natural Resources published an information sheet claiming that:
An ideal soil would be made up of 45% minerals (sand, clay, silt), 5% organic (plant and animal) material, 25% air, and 25% water. The mineral portion would be loam (20 – 30% clay, 30 – 50% silt, and 30 – 50% sand). It would be crumbly, relatively dark in color, smell earthy and rich, teem with microorganisms and earthworms, have plenty of nutrients, and have a pH between 6.5 and 7.5. (Londeree).
A lab soil test will indicate which nutrients your soil is deficient in and which minerals are in excess and suggestions for how to correct it.
Finally, amend garden soil before planting
Once you have your soil test results, you can begin amending your soil! The best time of the year to amend soil is in early spring before you plant anything. As soon as the soil is workable, you can till or hand cultivate your garden beds and add in the amendments.
Amending the soil is an ongoing process—ideally, you will reapply the needed soil amendments once a year at the beginning of the growing season. If you have the time and the resources, you can also apply some amendments in between different successions of crops, especially if those plants are heavy feeders, like broccoli and cabbage. Fall is another great time to add soil amendments and plant a cover crop since you’ll be building good soil while allowing the earth to rest.
If you’re amending garden beds that are already planted, add amendments a little at a time. Don’t add more than five pounds of sulfur or lime per 100 square feet for an established garden. If you start from scratch, there’s no reason you can’t add the entire amount at once.
Conclusion
Adding organic and inorganic soil amendments can vastly improve your soil's health and fertility. To get the most out of any soil amendments, your soil must be tested to determine what nutrients are lacking. Then, you can add the right amount of amendments and tailor your strategy to fit your particular needs.
Finally, amend your soil at the right time for the best results. Spring and fall are ideal times to amend the soil, but you can also add amendments as needed in between your crops. With a good soil amendment strategy and careful maintenance, you will have the garden of your dreams in no time.
